1. Field of the Invention:
The invention relates to a method and a device for the nondestructive material testing by means of ultrasonics on generator rotor teeth having an inserted slot wedge therebetween, using at least one transmit probe and at least one receive probe.
2. Description of the Prior Art:
Generator rotor teeth consist of high-tensile steel and are subjected during operation to a consistently high mechanical loading by forces acting in the axial and radial directions and resulting, on the one hand, from the so-called alternation of rotational load and, on the other hand, from the contact pressure of the slot wedges against the rotor teeth flanks.
Under unfavorable conditions and with the additional occurence of frictional corrosion at the areas of contact between the slot wedge and the rotor tooth, fatigue cracks can form which originate from these contact areas. These fatigue cracks are always oriented vertically with respect to the rotor axis and, under the above-mentioned mechanical stresses, can continue to grow after a certain critical crack depth has been reached. These defects in the rotor teeth would present a considerable safety hazard for the operation of the electric machine. For this reason, a regular check of rotor teeth is very important. In order to avoid high disassembly and assembly costs and prolonged outages of the electric machine, the rotor teeth are tested in the wedge-slot condition.
For this test, the nondestructive ultrasonic method of testing materials is advantageously used.
In the "Technical Disclosure Bulletin" of the "Central Electricity Generating Board, London," No. 330, of July 1979, entitled "The In-Situ Ultrasonic Examination of Large Generator Rotors," such a test method is described and illustrated. However, this test method can apparently only be used to test rotor teeth where the wedge support areas or tooth flanks, respectively, run parallel to the tooth backs of the rotor teeth.
Most of the rotor designs, however, have wedge support areas which are not parallel to the tooth backs of the rotor teeth but which normally form an angle of 40.degree.-70.degree. with the vertical of the rotor tooth neck. The above-mentioned test method cannot be used for this type of rotor design.